Superweeds immune to herbicides

By STACI ANN WILSON WRIGHT, Ledger staff writer | Jul 05, 2013

As if contending with drought, high temperatures and flood conditions weren’t enough to make an Iowa farmer’s job difficult, there may soon be an even greater threat to crops in the state – the superweed.

Labeled a superweed because it grows faster, produces more seed, and is almost completely resistant to herbicides, Palmer Amaranth already is posing a huge challenge for cotton and soybean farmers in the south. Characterizing the weed as “waterhemp on steroids,” Mark Carlton, field agronomist for Iowa State University Extension and Outreach, said the only proven method of controlling Palmer Amaranth is to remove it from the ground.

“That’s what they are having to do in the south,” Carlton said. “They are hiring people to go out into their fields, and they are physically hoeing it out.”

The even worse news is that Palmer Amaranth is on the move, spreading north quickly. The superweed already has been identified in Missouri, Southern Illinois, Nebraska and Wisconsin. Carlton said it is merely a matter of time before the superweed is identified in Iowa.

“It’s coming,” he said. “It’s not ‘if,’ it’s ‘when.’”

While it appears inevitable that corn and soybean farmers across the state will battle Palmer Amaranth in the near future, they are already battling a host of other herbicide-resistant weeds, Carlton said. Waterhemp is currently the most problematic herbicide-resistant weed, but horse weed, or marestail, giant ragweed and other weeds also are showing significant signs of herbicide resistance.

Most Iowa farmers over the last 10-12 years have relied on a method they once thought was fail-safe.

They have been spraying the herbicide glyphosate on fields planted with seeds that have been genetically altered to tolerate the chemical, Carlton said. Over time, the weeds have become immune to glyphosate.

“This is just like people and penicillin,” he said. “Over time, infections have become resistant to our common antibiotics. The same thing is happening with weeds. The mode of action we have relied on to kill them is not working because they have built up a resistance to it.

“Nature has the ability to survive, and over time we’ve seen we have survivors,” Carlton said. “We don’t just have problems with resistant weeds, but we also have problems with resistant insects and resistant pathogens that cause disease in plants.”

The problems, he said, are increasing at an alarming rate.

“Two years ago we saw a few weeds that wouldn’t die. Last year, we saw a few more, and this year, it’s worse,” he said. “We have several fields that are looking pretty ugly because of all the weeds growing out there.

“We know the fields have been sprayed but the weeds just didn’t die. The chances are, they aren’t going to die,” said Carlton. “I’m very concerned.”

Some agronomists have pointed the finger of blame at the Monsanto Company, a multinational agricultural biotechnology corporation headquartered in Creve Couer, Mo. Monsanto is the leading producer of genetically engineered (GE) seed and of glyphosate, which it markets under the Roundup brand. Carlton disagrees.

“The blame can lie with the farmer, the ag retailer and all of us in agriculture,” he said. “We could kill weeds [with glyphosate] that were huge. It was a great tool for about 10 to 12 years, but we took a great product out and we abused it.”

Just as farmers are responsible for the predicament they are in, farmers also hold the key to resolving the problem, Carlton said.

“Farmers are going to have to make a game plan,” he said.

That game plan will involve educating themselves about the growing problem of herbicide-resistant weeds and how to address it. ISU Extension and Outreach already is gearing programming toward finding solutions to statewide weed resistance.

“It’s going to be a hot topic at all our meetings,” Carlton said. “When farmers have the opportunity to come listen to our weed scientists speak, they, by all means, should do so.”

A second step in addressing weed resistance will be reevaluating herbicide programs by identifying every type of weed in the fields and then using a combination of herbicides to kill as many of them as possible.

Crops will more than likely require multiple applications of an herbicide blend, and the use of residual herbicides that prevent the weed from germinating in the first place will become more important than ever, Carlton said.

“Farmers are going to have to use a combination of products,” he said. “If we can use multiple modes of action, it increases the likelihood that we can kill the weeds.”

Early application of herbicide also will become more critical than ever before.

“When a weed gets above four inches tall, it gets very hard to control,” Carlton said. “Wet years like this one, which delay spraying, only compound the problem. We initially had some small weeds but due to the wet conditions, we couldn’t get in there in a timely fashion to spray. In a week’s time, a weed can go from four inches to twelve.”

Much to the chagrin of most farmers, eradicating these hardy weeds may mean a return to cultivation.

“Thirty years ago, it was quite common to see tractors out in the corn field or the bean field with a cultivator. The cultivator tills the ground in between the corn and soybeans, and the tillage kills the weeds,” he said.

A return to cultivating, Carlton acknowledged, will be time consuming and costly.

“When farmers are cultivating, they have to go very slow so they don’t hurt their crop. It takes a lot of time. Plus, it’s expensive. It’s another trip across the field. It’s more diesel fuel burned. It’s more equipment in the farmer’s line-up,” he said.

Labor expenses also could increase significantly.

“We are already seeing agronomists in other states suggesting that farmers or their hired help go out into their fields with a plain old hoe and walk the beans,” Carlton said.

When asked how corporate farms or family farms consisting of more than a thousand acres will address the potential need for manual weed removal, Carlton said, “I don’t know.”

“They are going to have to hire a lot of labor, I think,” he said. “They are going to have to get somebody to help them manage it. I have enough problems with my little 50 acres of corn, let alone 5,000. Honestly, I really don’t know how they will manage.”

As weeds continue to develop resistance to herbicides and as cross-pollination spreads the resistance, Carlton said the planting process will become increasingly complex.

“All of these things are just going to become part of the planting process,” he said. “Thirty years ago we could spray the weeds once and we were done. Now, Mother Nature has thrown us a curveball.”